Diagnostic Testing for EBV-Associated Myopathy
The diagnosis of EBV-associated myopathy requires demonstrating EBV DNA, RNA, or protein in muscle tissue via biopsy with immunohistochemistry (specifically EBER staining), combined with elevated muscle enzymes, serological evidence of EBV infection, and exclusion of other causes. 1, 2
Understanding the "FISH Test" Terminology
The term "Fish test" likely refers to EBER (Epstein-Barr virus-encoded small RNA) in situ hybridization, which is the gold standard for demonstrating EBV presence in tissue. 2 This is not fluorescence in situ hybridization (FISH) in the traditional sense, but rather an RNA in situ hybridization technique that detects EBV-encoded RNA in infected cells within muscle tissue. 2
Essential Diagnostic Workup
Muscle Enzyme Testing
- Creatine phosphokinase (CPK), LDH, AST, ALT, and aldolase should be measured as initial screening. 1
- Elevated muscle enzymes help differentiate true myositis from polymyalgia-like syndromes where CK levels remain normal. 1
EBV Serological Testing
- VCA-IgM, VCA-IgG, and EBNA-1 IgG provide the most reliable serological interpretation (>95% accuracy). 3
- Elevated IgG antibodies to VCA and EA (typically VCA-IgG ≥1:640 and EA-IgG ≥1:160) suggest chronic active EBV infection. 1
- Positive IgA antibodies to VCA and/or EA are often demonstrated in chronic active EBV. 1
- Note that 5-10% of EBV-infected patients never develop EBNA antibodies, which can complicate interpretation. 1, 3
Quantitative EBV DNA Testing
- Quantitative PCR on peripheral blood mononuclear cells should be performed. 1, 4
- More than 10^2.5 copies/mg DNA in peripheral blood mononuclear cells is generally detected in chronic active EBV infection. 1, 3
- This helps distinguish active infection from latent EBV (healthy individuals occasionally show low-level positivity). 1
Muscle Biopsy with EBER Staining
- Muscle biopsy is essential when myopathy presentation is atypical or when EBV-associated myositis is suspected. 1
- EBER immunohistochemistry staining must be performed on the muscle tissue to demonstrate EBV presence. 2
- The biopsy should target a weak muscle, ideally guided by EMG abnormalities or MRI findings. 1
- Expert histopathological interpretation is required to define inflammatory features. 1
Supportive Diagnostic Tests
- EMG shows myopathic changes (polyphasic motor unit action potentials of short duration and low amplitude with fibrillations). 1
- MRI of muscles using T2-weighted/STIR sequences can demonstrate muscle inflammation and guide biopsy site selection. 1
- Myositis-specific and myositis-associated antibodies should be checked to exclude other autoimmune myopathies. 1
Critical Differential Diagnoses to Exclude
Before confirming EBV-associated myopathy, exclude: 1
- Metabolic or mitochondrial myopathies (especially if presentation is atypical)
- Muscular dystrophies (limb-girdle, dysferlinopathies)
- Drug-induced myopathy (particularly statins)
- Endocrine myopathies (thyroid disorders, hyperparathyroidism)
- Other inflammatory myopathies (dermatomyositis, polymyositis)
Important Clinical Pitfalls
- EBV-associated myositis is extremely rare and can be fatal, particularly when associated with hemophagocytic syndrome. 2
- Performing limited immunohistochemistry with EBER on muscle biopsy is essential for diagnosis—standard muscle biopsy without EBER staining will miss EBV involvement. 2
- False-positive VCA-IgM can occur with CMV infection, leukemia, and other conditions, so confirmation with quantitative PCR is important. 3
- Chronic active EBV infection represents a failure to control the virus and has poor prognosis, particularly with late onset, thrombocytopenia, and T-cell infection. 1, 5
- Patients may develop hemophagocytic lymphohistiocytosis or lymphoproliferative disorders during the disease course, requiring monitoring for fever, cytopenias, hepatosplenomegaly, and coagulation abnormalities. 1, 2